Surface-treated steel plate and production method for same

ABSTRACT

The present invention relates to a surface-treated steel plate and to a production method for same, and more specifically relates to a surface-treated steel plate having outstanding corrosion resistance and thermal conductivity. In order to achieve this aim, in the present invention a macromolecular resin composition comprising graphene, which has highly outstanding physical properties including electrical conductivity, is coated onto the surface of a steel foundation plate, thereby making it possible to provide a surface-treated steel plate able to supremely well ensure not only corrosion resistance but also the intrinsic characteristics of graphene due to the macromolecular resin composition.

TECHNICAL FIELD

The present disclosure relates to a surface-treated steel sheet and amethod for manufacturing the same and, more specifically, to asurface-treated steel sheet having excellent corrosion resistance andthermal conductivity.

BACKGROUND ART

Generally, steel sheets for applications such as automobile panels aretreated with phosphates and are painted after a car body assemblyprocess in an automotive manufacturing plant. In this case, severalfolded portions may be present in steel sheets of an assembled car body.Since it is difficult for phosphates or paint to permeate into thefolded portions, the folded portions may be vulnerable to corrosion.

In the related art, a method of sealing the folded portions of steelsheets has been used to prevent corrosion-causing substances frompermeating into the folded portions. However, since the method decreasesthe productivity of final products and increases manufacturing costs,surface-treated steel sheets having high corrosion resistance and thusnot requiring sealing treatments have been in demand.

Recently, steel sheets formed by coating base steel sheets with organicfilms have been widely researched as types of surface-treated steelsheets not requiring sealing treatments. As a result, surface-treatedsteel sheets coated with organic films of a uniform thickness are beingcommercialized.

Such surface-treated steel sheets coated with the organic films ensurecorrosion resistance even in portions thereof which are not painted ortreated with phosphate, but the weldability thereof during electricresistance welding may be lowered due to the organic films beingrelatively thick.

Therefore, a surface-treated steel sheet having high corrosionresistance and excellent weldability during electric resistance weldingis required.

DISCLOSURE Technical Problem

An aspect of the present disclosure provides a surface-treated steelsheet having excellent weldability due to high thermal conductivity aswell as high corrosion resistance, and a method for manufacturing thesame.

Technical Solution

According to an aspect of the present disclosure, there is provided asurface-treated steel sheet including a base steel sheet and a polymerresin layer containing graphene on the top of the base steel sheet,wherein the graphene is arranged within the polymer resin layer.

According to another aspect of the present disclosure, there is provideda method for manufacturing a surface-treated steel sheet, including:preparing a base steel sheet; coating the top of the base steel sheetwith a polymer resin composition containing graphene; and drying andcuring the coated base steel sheet to form a polymer resin layercontaining graphene, wherein the graphene is arranged within the polymerresin layer by applying an electric field or a magnetic field to thebottom of the coated base steel sheet during drying.

Advantageous Effects

According to an exemplary embodiment of the present inventive concept, asurface-treated steel sheet is prepared by forming a polymer resin layercontaining graphene on the top of a base steel sheet so that it may havesuperior corrosion resistance to that of a steel sheet prepared by usinga conventional organic coating film, and may also obtain thermalconductivity.

In addition, according to another exemplary embodiment of the presentinventive concept, a surface-treated steel sheet may include a polymerresin layer containing graphene having a uniform arrangement andthickness.

BEST MODE

The inventors of the present invention researched a surface-treatedsteel sheet for applications such as automobile panels in order toobtain superior corrosion resistance and ensure superior weldabilityduring subsequent electric resistance welding to those of conventionalsurface-treated steel sheets, and discovered that in a case in which thesurface of a steel sheet is coated with a polymer resin compositioncontaining graphene having excellent properties such as electricalconductivity and mechanical strength so as to allow the graphene to havea uniform thickness, corrosion resistance and thermal conductivity maybe obtained from the uniformly arranged graphene and the inherentproperties of the graphene may also be imparted to the surface-treatedsteel sheet. Consequently, the inventors have completed the presentinventive concept.

Graphene is a material containing carbon atoms and provided in the formof a thin layer having a thickness of a single carbon atom and atwo-dimensional planar structure in which the carbon atoms are bondedtogether in a hexagonal honeycomb shape, and it is known that graphenehas excellent properties such as strength, electric conductivity,thermal conductivity, and electrical characteristics.

Presently, research is being actively conducted by utilizing theproperties of graphene in several industrial fields. For example, atechnique of coating a substrate or the like with a graphene solution isbeing used. The graphene solution coating technique facilitates thecoating of graphene in a simple way, but causes a problem in a case inwhich the coating of graphene sheets is not uniformly performed in thesame direction.

According to the present inventive concept, there is provided a novelstructure of a surface-treated steel sheet solving the aforementionedproblems through the arrangement of graphene toward an interface betweena base steel sheet and a resin layer or the surface of the resin layer.

That is, according to the research results of the inventors, sincegraphene is a carbon-based material and has a hexagonal sheet structureof carbon atoms through SP² bonding, it is able to effectively inhibit afine-scale growth on the surface of a cold-rolled steel sheet. Agraphene layer of a densely packed structure may effectively block thepermeation of moisture or the like even in a corrosive environment,thereby imparting corrosion resistance to the steel sheet.

Therefore, the present inventive concept provides the surface-treatedsteel sheet having excellent corrosion resistance and thermalconductivity, the surface-treated steel sheet including the base steelsheet and the polymer resin layer containing graphene on the top of thebase steel sheet, wherein the graphene sheets may be arranged inside thepolymer resin layer in a certain direction.

Here, an average thickness of the polymer resin layer containinggraphene formed on the top of the base steel sheet may be 0.5 to 5.0 μm,and more preferably 1 to 3 μm. When the average thickness of the polymerresin layer containing graphene formed on the top of the base steelsheet is less than 0.5 μm, the amount of graphene contained in thepolymer resin layer is not sufficient to provide the inherent propertiesof graphene, and it may be difficult to ensure the corrosion resistanceof the polymer resin layer itself. Conversely, when the averagethickness of the polymer resin layer containing graphene exceeds 5.0 μm,the inherent properties of graphene are sufficiently obtained, but thepolymer resin layer may be formed non-uniformly and the workabilitythereof in subsequent processing may be degraded.

A method for manufacturing a surface-treated steel sheet according to anembodiment of the present inventive concept is not particularly limitedso long as a polymer resin layer containing graphene can be formed onthe top of a base steel sheet. A method of preparing a graphene solutionin advance and applying the prepared graphene solution to the surface ofthe base steel sheet to form a graphene layer is generally used, butsuch a method may cause problems in regard to process efficiency,adhesion, and the like.

Therefore, an aspect of the present inventive concept may provide amethod of coating a base steel sheet with graphene sheets in a mannerthat the graphene sheets are arranged to have the same direction. Thatis, according to the present inventive concept, when the surface of thebase steel sheet is coated with a polymer resin composition containingthe graphene sheets, the graphene sheets contained in the compositionmay be arranged inside a polymer resin layer in a certain direction byusing an electromagnetic field (an electric field or a magnetic field),thereby improving adhesion to the base steel sheet.

That is, according to the research results of the inventors, hydroxylgroups (OH) present at the edges of graphene have high solubility, andthus, in a case in which a graphene solution coating technique is used,a large-area thin film may be formed, and the adhesion of graphene tothe base steel sheet may be improved by uniformly arranging the hydroxylgroups (OH) of graphene. In addition, the inherent properties ofgraphene may be effectively imparted thereto.

Hereinafter, examples of the method for manufacturing a surface-treatedsteel sheet, according to embodiments of the present inventive concept,will be detailed. The inventive concept may, however, be exemplified inmany different forms and should not be construed as being limited to thespecific embodiments set forth herein. While those skilled in the artcould readily devise other varied embodiments through the addition,modification or deletion of elements, such embodiments may fall withinthe scope of the present inventive concept.

Hereinafter, a method for manufacturing a surface-treated steel sheetincluding a polymer resin layer containing graphene, according to anexemplary embodiment of the present inventive concept, will be detailed.

A method for manufacturing a surface-treated steel sheet having apolymer resin layer containing graphene formed on the surface thereof,according to an exemplary embodiment of the present inventive concept,may include: preparing a base steel sheet; coating the top of the basesteel sheet with a polymer resin composition containing graphene; anddrying and curing the coated base steel sheet. During drying, anelectric field or a magnetic field may be applied to the bottom of thecoated base steel sheet.

The base steel sheet used in the method for manufacturing asurface-treated steel sheet, according to the embodiment of the presentinventive concept, is not particularly limited, and a general ferrous ornon-ferrous steel material may be used therefor.

After the base steel sheet is prepared as stated above, the top of thebase steel sheet may be coated with the polymer resin compositioncontaining graphene.

Here, the polymer resin composition containing graphene to be used maybe a composition in which graphene is contained in a general polymerresin composition. Any type of commonly used binder resin may be usedfor the polymer resin composition. Preferably, at least one resinselected from the group consisting of a urethane resin, an acryl resin,an epoxy resin, an ester resin, and an olefin resin may be used. Morepreferably, a urethane resin may be used. The polymer resin compositionmay further include a curing agent and a corrosion resistance enhanceras well as the binder resin.

Here, the content of the binder resin may be 10 to 90 parts by weight onthe basis of the total weight of the resin composition. In a case inwhich the content of the binder resin is less than 10 parts by weight,the salt water resistance of the binder resin (for example, the urethaneresin) and the chemical resistance thereof to the permeation of chemicalsubstances may not be exhibited, whereby the chemical resistance andalkali resistance may be lowered. As a result, when being degreased withan alkaline solution of pH 10 or above for five minutes at 60° C., theresin film may be discolored or delaminated. Conversely, in a case inwhich the content of the binder resin exceeds 90 parts by weight, thestability of the resin solution due to a congealing phenomenon may bedegraded, and manufacturing costs may be increased in an economicallyundesirable manner. Therefore, the resin composition may include 10 to90 parts by weight of the binder resin on the basis of the total weightof the resin composition, and water as the remainder.

A urethane resin which is usable as the binder resin has high waterresistance, chemical resistance, acid resistance, and alkali resistance,and a coated film of the urethane resin is soft and rigid, so that itmay be widely used to coat a steel sheet, an aluminum plate, or the likeso as to prevent surface scratching or to impart chemical resistancethereto. Therefore, any type of urethane resin commonly used in the artfor the aforementioned purposes may be used. However, such a urethaneresin has limitations in exhibiting softness and rigidity when usedalone. Accordingly, in the embodiment of the present inventive concept,the urethane resin may be used by combining a soft urethane-based resinand a hard urethane-based resin.

In this case, the content of the soft urethane-based resin may be 5 to95 parts by weight on the basis of a solid content of the urethaneresin. In a case in which the solid content of the soft urethane-basedresin is less than 5 parts by weight, workability may be improved, butthermal resistance and water resistance may be lowered. Conversely, in acase in which the solid content of the soft urethane-based resin exceeds95 parts by weight, there is no effect of improving workability, andcorrosion resistance may be significantly lowered.

The soft urethane-based resin may be a polyurethane resin prepared fromisoporene diisocyanate, dibasic acid, and polyhydric alcohol, such as apolyurethane dispersion resin or a polyethylene modified polyurethaneresin, or a polyurethane resin prepared from acryl polyol orpolyisocyanate, such as an acryl-urethane resin or a polyethylene-acrylmodified polyurethane resin. Here, the polyhydric alcohol may be acrylpolyol, polyester polyol, polyether polyol, polyolefin-based polyol, ora combination thereof. In addition, a molecular weight of the softurethane-based resin may be 5,000 to 300,000. When the molecular weightof the soft urethane-based resin is less than 5,000, workability may beconsiderably degraded. Conversely, when the molecular weight of the softurethane-based resin exceeds 300,000, the stability of the resinsolution may be degraded.

In addition, the hard urethane-based resin may be a polyurethane resinprepared from polycaprolactone polyol or polycarbonate polyol anddiisocyanate, especially, paraphenylene diisocyanate, a polyurethaneresin prepared from 4,4′-bis(ω-hydroxyalkyleneoxy)biphenyl andmethyl-2,6-diisocyanatehexanoate, or a polyurethane resin having anacetal bond. A molecular weight of the hard urethane-based resin may be200,000 to 2,000,000. When the molecular weight of the hardurethane-based resin is less than 200,000, there is no effect ofimproving workability. When the molecular weight of the hardurethane-based resin exceeds 2,000,000, the stability of the resinsolution may be reduced and the viscosity thereof may be increased,resulting in degraded workability. The hard urethane-based resin mayhave a Shore A hardness of 40 to 90 at the time of manufacturing a dryfilm. When the Shore A hardness is less than 40, there is no effect ofimproving workability. Conversely, when the Shore A hardness exceeds 90,the coated layer, i.e., the resin layer, is so hard that it may becrushed during processing, and thus, there is no effect of improvingworkability. Therefore, the urethane resin may have the aforementionedrange of hardness.

Meanwhile, an acryl resin may be used as the binder resin. Since theacryl resin has excellent high temperature/high humidity resistance,cold resistance, and workability, and is also inexpensive, it is widelyused for metal-surface treatment. As the acryl resin usable in theembodiment of the present inventive concept, an acryl-based resinsynthesized with a general monomer composition including a carboxylgroup sufficient to be solubilized may be used. The acryl-based resinmonomer may be methyl(meth)acrylate, ethyl(meth)acrylate,isopropyl(meth)acrylate, normalbutyl(meth)acrylate, isobutyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, hydroxypropyl(meth)acrylate,stearyl(meth)acrylate, or hydroxybutyl(meth)acrylate, but is not limitedthereto.

A molecular weight of the acryl resin may be 50,000 to 2,000,000. Whenthe molecular weight of the acryl resin is less than 50,000, there is noeffect of improving workability. Conversely, when the molecular weightof the acryl resin exceeds 2,000,000, the stability of the resinsolution may be reduced and the viscosity thereof may be increased,thereby resulting in degraded workability.

In addition, an epoxy resin may be used as the binder resin. The epoxyresin is widely used as a metal coating material due to excellentadhesion, corrosion resistance, and top coat paintability. The epoxyresin usable in the embodiment of the present inventive concept may be abisphenol A-type resin, a bisphenol F-type resin, a novolac resin, orthe like.

A molecular weight of the epoxy resin may be 500 to 25,000. When themolecular weight of the epoxy resin is less than 500, a crosslinkingdensity may be increased, and thus it may be difficult to ensureworkability. When the molecular weight of the epoxy resin exceeds25,000, it may be difficult to solubilize the epoxy resin, and as thecrosslinking density of a cured coated film is reduced, corrosionresistance may be lowered.

In addition, an ester resin may be used as the binder resin. The esterresin is widely used as a metal surface treatment agent due to excellentcurability, chemical resistance, thermal resistance, plasticity, andadhesion to an organic material. The ester resin usable in theembodiment of the present inventive concept may be a polyester resinprepared from maleic anhydride, isophthalic acid, terephthalic acid,tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, adipicacid or pimalic acid, an ethyleneglycol-modified ester resin, apropyleneglycol-modified ester resin, or a neophentylglycol-modifiedester resin.

A molecular weight of the ester resin may be 2,000 to 20,000. When themolecular weight of the ester resin is less than 2,000, a crosslinkingdensity may be increased, and thus workability may be degraded.Conversely, when the molecular weight of the ester resin exceeds 20,000,the cost is increased, and an increase in crosslinking density may leadto poor salt water resistance, resulting in degraded corrosionresistance.

In addition, an olefin resin may be used as the binder resin. The olefinresin has an effect of preventing scratches of a painted surface aftermetal surface treatment due to high water resistance, acid resistance,and salt water resistance. The olefin resin usable in the embodiment ofthe present inventive concept may be an aqueous polyolefin resin, andpreferably, a polyethylene resin, a vinyl-modified polyethylene resin, apolyvinylbutylene resin, a vinylchloride copolymer resin, a vinylacetatecopolymer resin, or a polyvinylalcohol resin.

A molecular weight of the olefin resin may be 50,000 to 2,000,000. Whenthe molecular weight of the olefin resin is less than 50,000, acrosslinking density may be increased, and thus it may be difficult toensure workability. Conversely, when the molecular weight of the olefinresin exceeds 2,000,000, it may be difficult to solubilize the olefinresin, and sedimentation of the resin occurs. Further, as thecrosslinking density of a cured coated film is reduced, corrosionresistance may be lowered.

The polymer resin composition containing graphene, according to theembodiment of the present inventive concept, may be a compositionincluding the polymer resin composition containing the binder resin and0.01 to 20 wt % of graphene, more preferably, 0.1 to 20 wt % ofgraphene.

The polymer resin composition may play a role of ensuring corrosionresistance of the base steel sheet. Graphene in the polymer resincomposition is provided to impart the inherent properties of graphene tothe base steel sheet. The base steel sheet may obtain excellent thermalconductivity, as well as excellent corrosion resistance, from thegraphene, thereby ensuring excellent weldability during processing.

As described above, in order to impart the inherent properties ofgraphene to the base steel sheet, the polymer resin composition forcoating the base steel sheet is required to contain a certain amount ofgraphene. In this case, after graphene powder is added to the polymerresin composition, they may be mixed with each other.

Here, the content of graphene contained in the polymer resin compositionmay be 0.01 to 20 wt %. When the content of graphene is less than 0.01wt %, the desirable graphene properties may be significantly limited.Conversely, when the content of graphene exceeds 20 wt %, there is nodifficulty in obtaining the graphene properties, but an excessive amountof graphene may be non-uniformly coated on the surface of the base steelsheet.

A method of coating the top of the base steel sheet with the polymerresin composition containing graphene is not particularly limited. Forexample, a method of using a bar-coater, a roll-coater or a curtaincoater may be used. As an example, in a case of a roll coating methodusing a roll coater, a composition to be coated (a graphene solution inan embodiment of the present inventive concept) may be put into a mixingtank, and it may be sufficiently stirred and transferred to a coater panthrough a circulation pipe. Subsequently, the composition may be appliedto the top of a plated layer by the roll coater in the coater pan,thereby coating the plated layer.

After the base steel sheet is completely coated with the polymer resincomposition containing graphene, it may be dried and cured. Here, ageneral drying method commonly known in the art may be used. Forexample, a hot air heating method, an infrared heating method, or aninduction heating method may be used. Subsequently, the coated solutionmay be sufficiently cured to form a hardened layer.

Meanwhile, in the embodiment of the present inventive concept, while thecoated base steel sheet is dried, an electric field or a magnetic fieldmay be applied to the bottom of the coated base steel sheet.

Applying the electric field or the magnetic field while drying thecoated base steel sheet is intended to improve the adhesion of graphenecontained in the coated solution, namely, the polymer resin composition.More specifically, the graphene may not be uniformly provided, but maybe distributed inside the polymer resin composition immediately afterthe coating of the base steel sheet with the polymer resin compositioncontaining graphene. The graphene may include functional groups(hydroxyl groups (OH)) at the edges thereof, and when the electric fieldor the magnetic field is applied thereto, it may attract or repel thefunctional groups of the graphene. In this manner, the distributedgraphene may be concentrated on the interface between the base steelsheet and the resin composition, namely, the lower portion of the coatedpolymer resin composition or the upper portion thereof.

For example, while the coated base steel sheet is dried, a voltage of−200V to +200V (except for 0V) may be applied to the coated base steelsheet. Since the OH groups of the graphene have a negative (−) electriccharge, when a positive (+) voltage is applied to the base steel sheet,the OH groups of the graphene may be arranged in a direction toward thebase steel sheet, and when a negative (−) voltage is applied thereto,the OH groups of the graphene may be arranged in an opposite direction,whereby adhesion may be improved. Here, the graphene may be present inthe form of sheets or may form a concentrated layer. In a case in whichthe graphene is present in the form of sheets, it may be advantageous toimpart the inherent properties of graphene to the steel sheet.

In order to achieve the aforementioned effects, the level of voltageapplied to the base steel sheet may change depending on the types ofbase steel sheets. A voltage range is not particularly limited, and avoltage of −200V to +200V (except for 0V) may be applied thereto.

Therefore, after the resin composition containing graphene is applied tothe top of the base steel sheet, when a voltage of −200V to lower than 0is applied thereto during drying, the graphene inside the subsequentlyformed polymer resin layer may be present in a direction toward thesurface of the polymer resin layer opposite the base steel sheet, andwhen a voltage of higher than 0 to +200V is applied thereto, thegraphene inside the subsequently formed polymer resin layer may bepresent in a direction toward the base steel sheet, namely, the lowerportion of the polymer resin layer.

Therefore, in order to apply the voltage to the coated base steel sheetwhile drying the same as described above, a drying device having adirect current (DC) power supply may be used.

The surface-treated steel sheet prepared according to the embodiment ofthe present inventive concept may have the inherent properties of thegraphene, namely, excellent thermal conductivity and the like, as wellas excellent corrosion resistance. Therefore, in a case of subsequentlywelding the surface-treated steel sheet, weldability may be obtained.

MODE FOR INVENTION

Hereinafter, the present inventive concept will be detailed throughexamples. However, these examples are provided so that this inventiveconcept will be more completely understood, and are not intended tolimit the scope of the inventive concept. The scope of the inventiveconcept is determined based on the matters claimed in the appendedclaims and modifications rationally derived therefrom.

Example

A base steel sheet formed of carbon steel was coated with a solutionincluding a polymer resin composition containing a polyurethane resin asa binder and 1 wt % of graphene using a bar coater. While the coatedbase steel sheet was dried using a hot air heating method, a voltage of−200V, 0V, or +200V was applied to the bottom of the base steel sheet,and consequently, a surface-treated steel sheet was manufactured.

In order to evaluate adhesion between the base steel sheet and thepolymer resin layer containing graphene in each surface-treated steelsheet manufactured by changing the applied level of voltage, thesurface-treated steel sheet was cross hatch cut into slices (1*1 mm) tentimes, and then an adhesion test was carried out through the attachmentand detachment of tape. The test results are illustrated in table 1below.

The adhesion was evaluated on the basis of the extent to which thepolymer resin layers were delaminated from 100 square slices by thetape. Here, in a case in which the resin layer was not delaminated dueto strong adhesion, it was evaluated as 100/100, while in a case inwhich the resin layer was entirely delaminated due to poor adhesion, itwas evaluated as 0/100.

In addition, corrosion resistance was also tested for eachsurface-treated steel sheet, and the test results are illustrated intable 1 below.

The evaluation of corrosion resistance was made by a salt water sprayingtest on the basis of the elapsed time until an area of white rustbecomes 5% of the surface area of the slice.

TABLE 1 Applied Voltage −200 V 0 V +200 V Adhesion Evaluation 90/10095/100 100/100 Corrosion Resistance 72 Hr 48 Hr 72 Hr Evaluation

As illustrated in table 1, according to the embodiment of the presentinventive concept, in the case of applying the polymer resin compositioncontaining graphene to the base steel sheet and applying the voltagethereto, excellent corrosion resistance as well as excellent adhesionbetween the base steel sheet and the resin layer may be obtained.

1. A surface-treated steel sheet comprising: a base steel sheet; and apolymer resin layer containing graphene on the top of the base steelsheet, wherein the graphene is arranged within the polymer resin layer.2. The surface-treated steel sheet of claim 1, wherein the graphene isdisposed within the polymer resin layer toward the base steel sheet or asurface of the polymer resin layer.
 3. The surface-treated steel sheetof claim 1, wherein the graphene is present in the form of aconcentrated layer or a sheet within the polymer resin layer.
 4. Thesurface-treated steel sheet of claim 1, wherein the polymer resin layercontaining the graphene has an average thickness of 0.5 to 5.0 μm. 5.The surface-treated steel sheet of claim 1, wherein the polymer resinlayer containing the graphene is formed of a composition including apolymer resin composition and 0.01 to 20 wt % of the graphene, and thepolymer resin composition includes: 10 to 90 parts by weight of at leastone resin selected from the group consisting of a urethane resin, anacryl resin, an epoxy resin, an ester resin, and an olefin resin as abinder resin on the basis of a total weight of the resin composition;and water as the remainder.
 6. A method for manufacturing asurface-treated steel sheet, comprising: preparing a base steel sheet;coating the top of the base steel sheet with a polymer resin compositioncontaining graphene; and drying and curing the coated base steel sheetto form a polymer resin layer containing the graphene, wherein thegraphene is arranged within the polymer resin layer by applying anelectric field or a magnetic field to the bottom of the coated basesteel sheet during drying.
 7. The method of claim 6, wherein the polymerresin composition containing the graphene comprises: the polymer resincomposition including 10 to 90 parts by weight of at least one resinselected from the group consisting of a urethane resin, an acryl resin,an epoxy resin, an ester resin, and an olefin resin as a binder resin onthe basis of a total weight of the resin composition, and water as theremainder; and 0.01 to 20 wt % of the graphene.
 8. The method of claim6, wherein a voltage of −200V to 200V (except for 0V) is applied to thebottom of the coated base steel sheet during drying.
 9. The method ofclaim 6, wherein the graphene is present within the polymer resin layertoward a surface of the polymer resin layer when cured after a voltageof −200V to lower than 0 is applied to the bottom of the coated basesteel sheet during drying.
 10. The method of claim 6, wherein thegraphene is present within the polymer resin layer toward the base steelsheet when cured after a voltage of higher than 0 to +200V is applied tothe bottom of the coated base steel sheet during drying.